The present invention relates to a method for coloring photochromic glasses, and more particularly to a method for influencing the surface coloration induced in such glasses by heat treatment in a reducing environment.
Photochromic glasses may be generally characterized as glasses which darken upon exposure to actinic radiation, e.g., ultraviolet light, and which fade in color upon the removal of the activating light. Such glasses have been widely employed commercially in the manufacture of ophthalmic lens blanks which will darken under bright outdoor conditions and fade in shade or indoors. Sunglass lenses have also been made of photochromic glasses of this type.
All of the commercially important photochromic glasses are glasses which contain a precipitated microcrystalline silver halide phase as the reversibly darkenable phase which causes the darkening of the glass. U.S. Pat. No. 3,208,860 provides the basic description of this family of glasses. Subsequent work has resulted in the development of many new families of photochromic glasses exhibiting faster darkening and/or fading response. U.S. Pat. No. 4,190,451, for example, provides a description of significantly improved photochromic glasses of this type.
Silver halide-containing photochromic glasses which exhibit a desired fixed tint in the undarkened state have also been commercially successful. These include glasses containing conventional glass colorants, as well as glasses wherein coloration is imparted not by the inclusion of glass colorants, but by treatment of the glass after manufacture with a coloring surface treatment. U.S. Pat. Nos. 3,892,582 and 3,920,463 disclose thermal reduction treatments useful for imparting yellow surface colors to photochromic glasses, while U.S. Pat. No. 4,240,836 describes a modified thermal reduction treatment which permits the development of a broad range of colors in this type of glass.
It is also known that red and yellow colors can be generated in the surfaces of certain alkali borosilicate glasses by so-called "staining" processes. U.S. Pat. No. 2,075,446, for example, teaches the such colors can be obtained by exchanging silver or copper ions for alkali metal ions in these glasses, followed by a thermal reduction post-treatment to develop the desired color. However, for these glasses, thermal reduction post-treatment are not effective to achieve other than yellow or amber colors in glasses which have been ion-exchanged with silver.
A coloration mechanism which has been postulated for the case of thermal reduction of photochromic glasses involves the growth of light-absorbing silver metal particles on or near the silver halide crystallites in the glass. Changes in the size and/or configuration of these particles may be responsible for the variations in coloration which are observed. In contrast, silver and/or copper "staining" coloration is attributed to precipitated metal particles alone.
In U.S. Pat. No. 4,537,612, a method for helping to control the coloration induced in a silver halide-containing photochromic glass by a thermal reduction treatment is described. In that patent, a silver halide-containing photochromic glass to be colored is subjected to an alkali metal ion exchange treatment during which potassium and/or lithium ions in the glass are exchanged for sodium ions from a suitable ion source such as a sodium salt. This alkali metal ion exchange, whether conducted prior to or subsequent to the coloring thermal reduction treatment, somehow alters the absorption bands generated in the glass by the treatment. Frequently, the result is a desirable shift in absorption to somewhat longer wavelengths, so that colors not readily generated by thermal reduction treatments alone can be developed in certain glasses.
Unfortunately, ion exchange processes such as described in the above-mentioned patent have not been found effective to modify the coloration of some well known commercial photochromic glasses to a useful degree. In particular, Corning Code 8097 glass, a glass commercially sold under the trade name PHOTOGRAY, is only slightly affected by these ion-exchange treatments.
It is therefore a principal object of the present invention to provide a modified method for developing surface coloration in photochromic glasses which offers improved effectiveness for treatment of photochromic glasses resistant to thermal reduction coloration.
It is a further object of the invention to provide a method for surface-coloring photochromic glasses which offers additional control over the hue and depth of coloration achievable by the thermal reduction of such materials.
Other objects and advantages of the invention will become apparent from the following description thereof.